Setting this attribute on a cookie marks it as non-cross-site, so it
is only send in requests to our own server. It is reasonable that no
one else should need our session or csrf data. Setting it explicitly
also prevents any issues from the ongoing change in browser behaviour [0]
when it is unset.
Seasurf supports the SameSite attribute starting with v0.3. As nothing
obviously broke, I used the opportunity and updated all the way to the
most recent version.
The SeaSurf default for SameSite is already `Lax`, so it only needs to
be set for the session cookie.
[0] https://developers.google.com/search/blog/2020/01/get-ready-for-new-samesitenone-secure
